1. Technical Field
This invention relates generally to a method for controlling compression ignition combustion in a compression-ignition engine, and more particularly to such a method for controlling the combustion phasing in compression ignition engines by the use of separate fuels having different volatility characteristics.
2. Background Art
Compression-ignition engines have proven to be particularly beneficial for many applications, including both vehicular as well as stationary installations. In particular, homogenous charge compression ignition (HCCI) combustion has demonstrated extremely low levels of exhaust NOx emissions and high fuel efficiency. Homogenous charge compression ignition is a combustion mode in which a homogenous air-fuel mixture starts reaction in an engine combustion chamber when the auto-ignition temperature is reached within the mixture. The reaction is initiated throughout the entire mixture, i.e., multi-point ignition, and proceeds without a visible flame front. However, control over the combustion phasing must be achieved through means other than those used in convention Diesel or spark-ignition engines.
U.S. Pat. No. 5,875,743, granted Mar. 2, 1999, to Daniel W. Dickey, one of the co-inventors of the present invention, titled APPARATUS AND METHOD FOR REDUCING EMISSIONS IN A DUAL COMBUSTION MODE DIESEL ENGINE, and assigned to the assignee of the present invention, describes the control of diesel engine emissions in a diesel engine adapted for at least partial operation in an HCCI combustion mode. U.S. Pat. No. 5,875,743 proposes the use of two fuel injectors, and optionally a water injector, for respective fuel and water injection based on engine operating parameters indicative of engine speed and load. U.S. Pat. No. 5,832,880, granted Nov. 10, 1998, also to Daniel W. Dickey, for an APPARATUS AND METHOD FOR CONTROLLING HOMOGENOUS CHARGE COMPRESSION IGNITION COMBUSTION IN DIESEL ENGINES, and likewise assigned to the assignee of the present invention, also describes controlling HCCI combustion by water injection.
More recently, U.S. patent application Ser. No. 09/738,446 was filed on Dec. 15, 2000, by Stefan Simescu, Thomas W. Ryan, III, and Daniel W. Dickey, for ENGINE AND METHOD FOR CONTROLLING HOMOGENOUS CHARGE COMPRESSION IGNITION COMBUSTION IN A DIESEL ENGINE. Thomas W. Ryan, III and Daniel W. Dickey are co-inventors of the present invention, which is likewise assigned to the assignee of the present invention. This application is directed to the control of homogenous charge compression ignition combustion by water injection into the combustion chamber subsequent to sensing an operative characteristic representative of a first combustion stage in the HCCI combustion process.
However, there remain problems in providing precise control of HCCI combustion. More particularly, controlling the HCCI combustion process has presented two significant challenges:
1) Controlling the combustion phasing with respect to the top dead center (TDC) position of a piston reciprocatively disposed in the combustion chamber, and
2) Controllably extending the operating range of HCCI combustion into high engine loads.
The present invention is directed to overcoming the problems of controlling compression ignition combustion, and particularly HCCI combustion. In particular, it is desirable to have a dual fuel system in which the two fuels have different volatility characteristics so that the combustion phasing of a compression-ignition engine can be controlled by use of the fuel having the more appropriate volatility for a specific operating condition. It is also desirable to have such a dual fuel system in which two fuels having different volatility characteristics are produced from a single multi-component parent fuel, thereby eliminating the need for separate on-board storage of two different base fuels.
In accordance with one aspect of the present invention, a method for controlling compression ignition combustion in a compression-ignition engine includes separating a single multi-component parent fuel into first and second separate fuel components, the first component having a lower volatility than the second component. At least one engine operating parameter correlative of a desired combustion phasing of the compression ignition process is sensed. Separate respective amounts of the first and second fuel components are then controllably introduced into each combustion chamber of the compression-ignition engine in response to the sensed values of the engine operating parameter.
Additional features of the present invention include separating the single multi-component parent fuel into first and second separate fuel components by distillation of at least a portion of the parent fuel. Other features include controllably introducing the separate first and second fuels, either directly into the combustion chamber or into an intake air passageway upstream of the intake valves of the engine by either a single injection nozzle having two controlled fuel passageways or through separate injectors.
In accordance with another aspect of the present invention, a method for controlling compression-ignition engine combustion in a compression-ignition engine adapted for homogenous charge compression ignition includes separating a single multi-component parent fuel into first and second separate fuel components in which the first fuel component has higher volatility characteristics than the second fuel component. At least one engine operating parameter correlative of a desired combustion phasing of the homogenous charge compression ignition process is sensed and respective separate amounts of the first and second fuels are introduced into the combustion chamber of the engine in response to the sensed engine parameter.
Yet another feature of the present invention includes the sensing of at least one engine operating parameter representative of the start of combustion of a first stage in the homogenous charge compression ignition combustion process.